Polyethylene terephthalate (PET) is a polyester polymer that is produced commercially, e.g., in a two step polymerization process. See: Kirk-Othmer, Encyclopedia of Chemical Technology, Third Edition, Vol.18, John Wiley & Sons, New York, N.Y. (1982) p. 534-536. This process generally comprises an esterification step and a polycondensation step. The esterification step may be either: a monomer formation by an ester interchange of dimethyl terephthalate (DMT) with ethylene glycol; or esterification of terephthalic acid (TA) with ethylene glycol. In either case, a monomer called bis-(2-hydroxyethyl) terephthalate (BHET) is formed.
During the polycondensation step, the monomer (BHET) is heated which causes the monomer to condense with itself to form polyethylene terephthalate. As the monomer condenses, ethylene glycol is liberated and removed, e.g. under vacuum. The material removed by vacuum ("overhead") contains ethylene glycol as well as some dimethyl terephthalate (DMT), monomer, and polycondensation catalyst. Typically, this overhead is stripped of excess ethylene glycol leaving a bottoms stream containing some residual ethylene glycol, monomer, and oligomer as well as the catalyst (e.g., 0.2 to 1% antimony, manganese, titanium, phosphorus).
Processes for the recycling of PET waste streams are known. "Methanolysis" is one such process. In methanolysis, the PET material is reacted with methanol to produce dimethyl terephthalate (DMT). For example, see U.S. Pat. Nos. 2,884,443; 3,037,050; 3,148,208; 3,321,510; 3,488,298; 3,907,868; 4,163,860; 4,578,502; and 5,051,528. "Glycolysis" is another process for recycling PET production waste. In "glycolysis", PET scrap is reacted with ethylene glycol to produce bis-(2-hydroxyethyl) terephthalate (BHET), the PET monomer. For example, see U.S. Pat. No. 4,078,143, column 1.
In the effort to recover excess ethylene glycol from the overhead of the polycondensation process, alkali metal compounds may be added to saponify the residual DMT and thereby improve the ethylene glycol recovery efficiency. It may be added for other reasons, which are incorporated herein by reference, see U.S. Pat. Nos. 4,118,582; 4,154,921. The addition of alkali metal compounds results in the formation of alkali metal salts of terephthalic acid, hydroxyethylterephthalate, or higher oligomer having an organic acid functional group. These salts lower the efficacy of the known recycling processes because they are inert in these processes.
Accordingly, there is a need for a process to convert these organic salts to a form that will react in recycle processes, and thereof increase the efficacy of those processes.